They are entiry different panels. What is your mission VFR or IFR? If VFR, I wouldn't be looking at any GTN. If IFR, I couldn't get myself to part with the extra $ for a 750.
I have no idea of the size plane this panel is in, but here is my take on screens. I love gadgets just like everyone else, but 3 10" screens..? I currently fly behind one SV 10" and a 430. I fly IFR without issue with the single screen (have a D6 backup with backup batteries in both). I would prefer a second screen to dedicate to mapping/charts/plates and will definitely install 2 in the RV-10 I'm building, but I can't see what the 3rd is used for sitting in front of the passenger..? I recently saw a gorgeous, 3-screen panel, but it left no room for independent backup AI. I guess you can say multiple ADAHRS is the backup, but I prefer a separate unit.
As far as IFR navigator, with some of Dynon's recent upgrades adding to GNS/GTN integration, I use my 430 very little during flight - initially loading a flight plan and loading an approach is about it. For me the 750 doesn't get me enough for the increased cost. That said - and it has been a while since I looked - I believe both can support remote comm 2, but only 750 supports remote comm panel..?